Alumina calcination

One advantage of sintering is the close control of si2e and shape of the abrasive particle. Extmded, cylindricaHy shaped, sintered abrasives of circular cross section were produced from bauxite (24) and from calcined alumina (25). The Uelt2 sintered bauxite was also later produced in extmded cylinder form and designated as 76A. Extmded sintered abrasives of a wide variety of cross-sectional configurations, eg, square and triangular, were later patented (26). 

Sintered abrasives made from bauxite and calcined alumina are heavy-duty abrasives they are much too strong and tough for precision grinding. 

Calcined alumina, a-Al202, and naturally occurring comndum are practically insoluble in acids and bases, but partially calcined and low temperature amorphous oxide, such as that which forms on nacent commercial aluminum surfaces, is soluble... 

Specialty alumina derived from the Bayer process resemble SGA, except for a higher a-Al202 content, which is usually >80%, and a lower surface area, typically <20 m /g. The remaining material is generally sodium P-alurnina (Table 2) formed as a result of the soda contamination common to the process (2—7). The mineralogical and stmctural properties of calcined aluminas are given in Table 2 (3). 

Specialty Aluminas. Process control (qv) teclmiques permit production of calcined specialty aluminas ha nng controlled median particle sizes differentiated by about 0.5 ]lm. Tliis broad selection enables closer shrinkage control of high tech ceramic parts. Production of pure 99.99% -AI2O2 powder from alkoxide precursors (see Alkoxides, metal), apparently in spherical form, offers the potential of satisfying the most advanced appUcations for calcined aluminas requiring tolerances of 0.1% shrinkage. 

Calcined alumina markets consume slightly less than 50% of the specialty alumina chemicals production (1—8,20,22—115). Worldwide usage is estimated to be about 50% for refractories (qv), 20% for abrasives, and 25% for ceramics (qv). Calcined aluminas are also used in the manufacture of tabular alumina and calcium aluminate cements (CAC). Quantities are estimated to be over 200,000 and 100,000 t, respectively (7). 

Ceramics. Calcined aluminas are used in both electronic and stmctural ceramics (see Advanced ceramics). Electronic appHcations are dominant in the United States and Japan whereas mechanical appHcations are predominant in Europe (1). Specialty electronic integrated circuit packages generally use the low soda and thermally reactive aluminas. 

Calcined aluminas are also used for polishing appHcations by mixing into polishing compounds in the form of paste or suspensions. Polishing aluminas are used to alter the surfaces of metals, plastics, glass, and stones in the manufacture of cutlery, automobiles, computers, furniture, eyewear, semiconductors, and jewelry. Polishing aluminas are also used to coat surfaces, such as video tapes (1). 

Refractories. Calcined alumina is used in the bond matrix to improve the refractoriness, high temperature strength/creep resistance, and abrasion/corrosion resistance of refractories (1,2,4,7). The normal, coarse (2 to 5 )J.m median) crystalline, nominally 100% a-Al202, calcined aluminas ground to 95% —325 mesh mesh are used to extend the particle size distribution of refractory mixes, for alumina enrichment, and for reaction with... 

Refined calcined alumina is commonly used in combination with high purity limestone [1317-65-3] to produce high purity calcium aluminate cement (CAC). The manufacture, properties, and appHcations of CAC from bauxite limestone, as weU as high purity CAC, has been described (104). High purity CAC sinters readily in gas-fired rotary kiln calcinations at 1600 —1700 K. CAC reactions are considered practically complete when content of free CaO is less than 0.15% andloss on ignition is less than 0.5% at 1373 K. 

Calcined alumina is a reactive powder used to make synthetic grain. It also may be used as a bonding or fine component in batched refractory mixes, as a raw material for molten cast refractories, or for refractory casting sHps. 

Calciners alumina dust losses are controlled by ESPs SOj and NO, emissions are reduced to acceptable levels by contact with the alumina. 

Alumina, 2 345t 5 582. See also Activated alumina Aluminum oxide (alumina) Bauxite(s) Calcined alumina Fused alumina Tabular alumina in the activated catalyst layer, 10 41 adsorption capacity vs. years of service, 1 630... 

ASTM test methods for, 21 512t calcined alumina applications, 2 413-414 chrome, 21 518... 

Figure 4 in Ref. 216, reproduced on the right, displays Pt 4d5/2 XPS spectra from calcined alumina-supported platinum catalysts, pure (Pt/A) and doped with lanthanum (Pt/A-L), cerium... 

Zinc oxide (ZnO) is widely used as an active filler in rubber and as a weatherability improver in polyolefins and polyesters. Titanium dioxide (TiOj) is widely used as a white pigment and as a weatherability improver in many polymers. Ground barites (BaS04) yield x-ray-opaque plastics with controlled densities. The addition of finely divided calcined alumina or silicon carbide produces abrasive composites. Zirconia, zirconium silicate, and iron oxide, which have specific gravities greater than 4.5, are used to produce plastics with controlled high densities. 

Paige, B. E., "Leachability of Glass Prepared from Highly Radioactive Calcined Alumina Waste," National Reactor Testing Station, Rept. IDO-14672, (February, 1966). 

In this paper, we report the synthesis of mesoporous silica and alumina spheres with nanometer size (80 to 900 nm) in the present of organic solvent with aqueous ammonia as the morphological catalyst to control the hydrolysis of tetraethyl orthosilicate (TEOS) and aluminum tri-sec-butoxide.1181 Mesoporous silica spheres show hexagonal arranged pores with monodispersed pore sizes ( 2.4 nm) and high surface areas ( 1020 m2/g) similar to MCM-41. A large pore ( 10 nm) mesoporous alumina sphere templated by triblock copolymer is thermally stable. Calcined alumina sphere shows disordered mesoporous arrays with relatively uniformed pore size distribution and high surface areas ( 360 m2/g). 

TEM images (Figure 6) for calcined alumina spheres show that the spheres have non-uniformed size (average 400 nm) and disordered mesoporous arrays with relatively uniformed pore structure. 

Nitrogen adsorption/desorption isotherm for calcined alumina sphere (Figure 4b) is a type IV with a large hysteresis. A steep increasing occurs in the isothem curve at a relative pressure 0.55

narrow pore size distribution at the mean value of 10.0 nm. Calcined alumina sphere has BET surface area of 360 m2/g and pore volume of 0.62 cm3/g. 

Cement, dry process Cement, wet process, 44% water Limestone calcination Dolomite calcination Alumina preparation Barium sulfide preparation Ignition of inorganic pigments Iron pyrite roasting... 

According to M. Ascoli,9 liquid ammonia shows the phenomenon of electrosmose. When a difference of potential is established between two portions of liquid ammonia separated by a porous septum of calcined alumina, and kept at a temp, of —60° by immersion in a bath of acetone cooled with solid carbon dioxide, there is usually a slight displacement of the liquid in the opposite direction to the current but if the liquid is coloured blue by the formation of a small quantity of sodammoniuin in soln. there is a marked displacement of the liquid in the same direction as the current, showing that the introduction of the sodium has caused the liquid to become positively and the alumina negatively electrified. 

Molybdenum and cobalt on calcined alumina catalysts are partly extractable by washing. The amount of Mo removed by extraction with... 

Meiklejohn A, Jones WW. 1948. The effect of the used of calcined alumina in china biscuit placing on the health of the workmen. J Ind Hyg Toxicol 30 160-165. 

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